The present invention relates to a mast for a sail, more specifically for the sail of a sail-craft rigged with only one "cat"-type main-sail, but having no foresail, such as sail-boards or small yawls.
The most simple and common masts of this type have a round or roundish cross-section, often tapering towards the top end. In many cases, the non-braced masts of these small sail-craft are of hollow, tubular shape and made from fiberglass-reinforced resin. It must be noted that the chosen diameter for theses masts always represents a compromise. On one hand, a stiff mast, calling for a large diameter cross-section, is desirable, because it will not deflect much under high wind-pressure acting upon the sail, thus preserving an efficient sail curvature for maximum thrust. On the other hand, a slim mast with a small diameter would cause much less aerodynamic drag and turbulence. Such turbulence and drag can be especially harmful in the forward section of the sail, where an unimpeded airflow will provide most of the forward thrust. By using aluminum alloy instead of fiberglass, the stiffness will increase, however, at the same time the breaking strength will drastically diminish. By making masts instead from carbon-fibre-reinforced resin, Kevlar or other exotic materials, fairly slim masts with adequate breaking strength can be made. However, their price will be extremely high.
In order to try to improve the aerodynamic shape of theses masts, somewhat streamlined cross-sections, either oval or tear-shaped, have been manufactured, especially for masts being held in place and being supported by stays and guys, which will absorb practically all forces taking effect upon the mast. Thus, it is possible to employ much slimmer mast profiles than would be the case with non-braced masts.
When masts with tear-drop-shaped cross-sections are mounted firmly without means for turning around their own axis in order to point in the same direction as the sail, their frontal area exposed to the wind may even be larger than with simple round masts. Tear-drop-shaped masts only represent an improvement if they are allowed to turn in the same direction as the sail. Such articulated, tear-drop-shaped masts are being used on high-performance sail-crafts, such as catamarans, land-sailers and ice-boats or, in other words, where the resulting advantages will be most noticeable due to the potentially high speeds of these crafts.
Light sail-craft and sail-boards most often are equipped with free standing masts without guys or stays. These masts are usually hollow for weight reasons, and their cross-sections are either mostly round or have a a length-to-width ratio of approximately 1:1. Therefore, these masts have approximately the same flexibility and strength in all directions.
Airflow has a similar effect on a sail-system as on rigid air-foils. In both cases, the resulting force is called "lift". It is the result of the air flowing over and under the air-foil or sail, thereby changing the direction somewhat. It is desirable to convert a given air-flow into as much lift as possible, and this can only be achieved by sustaining the least possible air resistance. Lift is a force working usually at about a right angle to the sail or air-foil. However, whereas in a rigid wing, this force is trying to bend the center beam of the wing structure at a right angle to the wing surface, the sail mast will bend parallel with the sail or, in other words, approximately in the direction the wind is blowing. There is practically no loss of lift when a rigid wing bends under load. However, when a strong wind blows into a sail, it bulges and pulls the mast backwards, thereby causing the deepest point of the sail curvature to move backwards where it increases drag. Judging from the way masts are shaped, reinforced and braced, it must be concluded that it is common belief that normally masts are submitted to forces parallel to the lifting force. However, this is truly not the case in the case of masts without a foresail. This invention is based on the assumption that the tension of the sail is the main force to which the mast is submitted when under sail, and this force acts in the direction the sail is pointing immediately behind the mast.